Hard to say. There's a strong tradition of quite considerable deference to judgments about national security, and there are people who are quite passionate about the effects on unit cohesion, etc., of out homosexuals serving in the military. I could see the Court finding that to be a real problem and a sufficiently compelling interest; I could also see the Court going the other way.

Here's a post on the subject from a former officer and law student, who is more optimistic than I about the policy being struck down.